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As far as the temp sending units, the ones with the dual push-on terminals were used on flathead V8's where two sending units were installed. They would connect a wire daisy-chained from the passenger side sender to the driver side sender. Apparently, the reading you received at the guage would be the highest of the two senders.
In 1954, when the first Y-block OHV engine came out, FOMOCO simply used the same old style temp senders even though they only needed one wire connection. To my knowledge, they kept using them through at least 1956.